There have been many suggestions in the prior art of deep bed filters, i.e. filters consisting of carefully graded layers of granular filter medium, which are provided with an upper surface layer of relatively large, light weight materials which can act as a buffer or strainer for removing large particles, flocculents, or high concentrations of contaminants which have a tendency to surface load and plug the upper exposed surfaces. The prior art suggestions have attempted to provide particles for the upper surface which have the characteristics of (1) water-wetability, (2) relatively large uniform size, (3) relatively low, controlled density, and (4) liquid impermeability to avoid density change during use.
My earlier patent U.S. Pat. No. 4,190,533 proposes the utilization of particles of various materials, such as hollow glass beads, or pellets of either nitrile rubber, polysulfide rubber or polyurethane as the upper surface materials. This patent also specifically states that the particle size should be substantially uniform, and the particles should be of uniform shape. According to the disclosure of U.S. Pat. No. 4,190,533, the granules must have a uniformity coefficient of substantially about and not less than 1, wherein the uniformity coefficient is defined as the number obtained by dividing the sieve opening, in millimeters, which will pass 60% of the granules, by the sieve opening in millimeters which will pass just 10% of the granules.
In my earlier co-pending applications, of which this application is a continuation-in-part, I have proposed the utilization of different types of granules, specifically particles which comprise silica beads dispersed in a matrix of cured cement. The particles as earlier proposed were molded to shape, were cylindrical, and were of substantially uniform size and shape, although the granules might vary slightly.
It has now been found that granules of substantially uniform size and shape, as proposed both in my earlier U.S. Pat. No. 4,190,533 and in my earlier co-pending applications, tend to agglomerate and stick together upon backwashing, so that the particles will cling together and move upwardly as a piston or agglomerated mass when the filter bed is expanded during backwash. As a result, there is minimal relative movement among the particles during backwash, and there is no scrubbing action between the individual particles tending to separate the particles from the contaminants which have been removed by the particles during filtration.